1. Field of the Invention
This invention relates to wireless communications and, more particularly, to a system and method for switching between base stations.
2. Description of Related Art
Wireless communications systems include conventional cellular communication systems which comprise a number of cell sites or base stations (BTS), geographically distributed to support transmission and receipt of communication signals to and from wireless or units which may actually be stationary or fixed. Each base station handles communications over a particular region called a cell, and the overall coverage area for the cellular communication system is defined by the union of cells for all of the base stations, where the coverage areas for nearby cell sites overlap to some degree to ensure (if possible) contiguous communications coverage within the outer boundaries of the system's coverage area.
When active, a wireless unit receives signals from at least one base station or cell site over a forward link or downlink and transmits signals to (at least) one cell site or base station over a reverse link or uplink. There are many different schemes for defining wireless links or channels for a cellular communication system, including TDMA (time-division multiple access), FDMA (frequency-division multiple access), and CDMA (code-division multiple access) schemes. In CDMA communications, different wireless channels are distinguished by different channelization codes or sequences that are used to encode different information streams, which may then be modulated at one or more different carrier frequencies for simultaneous transmission. A receiver can recover a particular stream from a received signal using the appropriate code or sequence to decode the received signal.
Due to the delay-intolerant nature of voice communication, wireless units in conventional cellular systems transmit and receive over dedicated links between a wireless unit and a base station. Generally, each active wireless unit requires the assignment of a dedicated link on the forward link and a dedicated link on the reverse link. Traditional data applications are typically bursty and, unlike voice communications, relatively delay tolerant. However, wireless communication systems are evolving that will support a variety of real-time data services, such as providing voice over Internet Protocol (IP) using data packets to carry the voice information.
In a well known data only evolution of the third generation CDMA standard (hereinafter referred to as 3G-1x EVDO), voice and data services are provided using separate frequency carriers. Data is transmitted over a time division multiplexed carrier at variable data rates. Specifically, measured signal to interference ratio (SIR) or carrier to interference ratio (C/I) at the receiver is used to determine a data rate which can be supported by the receiver. In 3G-1x EVDO, the wireless unit performs the rate calculation using measurements of a pilot signal broadcast from the base station and reports back the rate at which it is going to receive data from the base station on a data rate control (DRC) channel. The DRC channel is spread using a Walsh code assigned to the base station sending the downlink packets to the wireless unit and is only received by that base station. The base station receives the reported rate and sends downlink packets at the reported rate.
FIG. 1 shows a the high data rate (HDR) architecture 10 for a 3G-1x EVDO system. In this architecture 10, base stations (BTSs) 12a–b perform the function of interfacing to the wireless unit or access terminal 14 (AT) over the air interface 16 with the radio access system 17. Each BTS 12a–b contains the hardware and software to perform the digital signaling processing required to implement the HDR air interface and to communicate with the other components of the radio access network 17. The BTS also contains the radio frequency (RF) components required to transmit the RF signals carrying the data over the air and to receive RF signals from the AT 14. A backhaul network 18, which can be implemented using a router(s), terminates the backhaul interfaces from several BTSs. This function is required to allow routing of information received from the air interface 16 to a control point for a session, where frame selection can be performed. The network 18 also allows routing of data between the BTSs 12a–b. 
A mobility server 19 includes a controller 20 and a packet control function 24. The controller 20 provides signaling and traffic processing control for each session. These functions include session establishment and release (performed by a functional entity called the Overhead Manager (OHM), frame selection and Radio link protocol (RLP) processing and RLP and Signaling Manager. These are collectively referred to as the HDRC function. The packet control function (PCF) 24 provides the processing for a standard A10/A11 R-P interface 28 to the PDSN and allows the HDRC functions to interface to a packet data service node (PDSN) 32. The A10/A11 interface terminates all mobility management functions of the radio access network 17. The PDSN 32 terminates a point to point protocol (PPP) link protocol with the AT 14. The PDSN 32 maintains link layer information with the PCF, and routes packets to external packet data networks. A network management function 33 can handle billing, authentication and providing various services.
When performing communications on the uplink, the AT 14 will send data to BTSs 12a–b in the active set of the AT 14. The AT14 maintains a list of BTSs 12a–b referred to as the active set which includes the BTSs 12a–b with which the AT14 is in communication. The uplink data arrives at the various BTS 12a–b and are forwarded by these BTS 12a–b to the controller 20. The controller 20 selects a frame using some quality criteria among the received frames. The controller 20 will also assemble the layer 3 packet from the RLP frames. Then, the resulting layer 3 packet will be forwarded to the PCF 24 and later to PDSN 32 to be routed to the final destination. All the BTSs 12a–b in the active set of the AT 14 listen to the AT 14 on the uplink. The AT 14 selects the BTSs 12a–b which are in the active set based on downlink channel quality. Downlink channel quality is determined based on measurements of pilot signals transmitted from the BTSs 12a–b. When the AT 14 communicates with more than one BTS 12a–b at the same time, the AT 14 is in soft handoff with those BTSs.
In the downlink direction, soft handoff is not supported. The AT 14 performs RF measurements and selects, based on such measurements, which BTS the AT 14 is to receive downlink data from. Accordingly, the AT 14 will establish downlink data link with one BTS, for example with BTS 12a. Downlink packets arriving at PDSN 32 for the AT 14 are routed via the A10–A11 interface 28 to the PCF 24. The PCF 24 will route it via the controller 20 to the appropriate BTS 12a–b that the AT 14 is communicating with at that particular instant. When the AT 14 decides to switch to a new BTS, for example to BTS 12b, for downlink data communication, the AT14 will not send any frames on the DRC Channel to the existing BTS 12a. Instead, the AT 14 will start sending signals on a DRC Channel to the new BTS 12b it has selected. Such signals can be sent every 1.67 ms several (say N=3) times.
The BTS12a will timeout eventually and send a message to the controller 20. BTS 12b will have received multiple signals on the DRC channel from the AT 14 that indicate that the AT 14 has intended to switch to BTS 12b. The BTSs 12b then sends some signaling messages to the controller 20 to indicate that the AT 14 has selected the new BTS 12b. The delay between the time that the AT14 sends signals on the DRC Channel of the new BTS 12b by changing to a Walsh code associated with the new BTS 12b and the time that the new BTS 12b receives the first downlink frame from the controller 20 can cause data packets to be delayed or lost. Note, after the AT14 has switched to the new BTS 12b, the AT 14 is not receiving any downlink data from BTS 12b since the controller 20 is not aware that the AT 14 has switched to the BTS 12b and is still forwarding traffic to the old base station BTS 12a. While this delay may be okay for web-browsing type of applications, it is definitely not desirable for voice over IP (VoIP) or real-time applications. With a packetization interval of 20 ms, 5 voice packets can be missed with a break of 100 ms.
In a more detailed example, on the downlink, the AT 14 receives data from only one BTS 12a at any given time. The DRC (Data Rate Control) channel established on the air link 16 is used by the AT 14 to indicate to the Radio Access Network 17 the forward traffic channel data rate that should be used to send to the AT 14. The encoding used to send the DRC information also selects the best serving BTS for the forward traffic channel. The AT 14 selects a rate based on the carrier to interference (C/I) estimate of the best serving BTS. The supported forward traffic channel data rate is mapped to a 4 bit DRC symbol to be transmitted on the DRC channel. An 8-ary Walsh code corresponding to the best serving BTS is used to spread the DRC channel transmitted. Each DRC symbol corresponds to a forward traffic channel data rate. Each 8-ary Walsh code corresponds to a BTS in the active set. The mapping is defined by DRCCover. The AT 14 reports the DRC Symbol and the DRCCover on the DRC Channel, to indicate the required transmission rate on the forward traffic channel and the current BTS 12a. 
If the AT 14 decides to switch to a new BTS 12b, the AT 14 changes the DRCCover to that of the new BTS 12b and switches to receive downlink data packets from the new BTS 12b. The new BTS 12b receives the DRCCcover from the AT 14, and the new BTS 12b informs the controller 20 that the new BTS 12b has been selected by the AT14 to transmit downlink traffic by to AT 14. The controller 20 arranges to establish the new BTS 12b as the transmission point for the downlink traffic to the AT14 for the session. However, a delay, for example of 100 ms, can occur from the time that the AT14 reports the DRC Cover for the new BTS 12b and the time that the downlink traffic is forwarded to the new BTS 12b. During that delay, data packets that are forwarded to the old BTS 12a from the controller 20 are lost and/or delayed because the AT14 has already switched to the new BTS 12b. 
One solution to eliminate the delay in switching between BTSs on the downlink to provide a seamless virtual handoff is to let the controller 20 multicast downlink data to all base BTS 12a–b in the active list. That way, when the AT 14 picks a new BTS, the new BTS already has downlink data that it can send to AT 14 so there will not be any missing downlink data. However, such a solution is not too attractive because it is not uncommon to have 3–6 BTSs in the active list. Such a multicast solution increases the backhaul transport cost (between the controller 20 and the various BTS 12a–b).